Recent trends in analog product design include the combination of all typical functional blocks of an analog circuit inside a single chip. Ideally, analog chips have to be compatible with both the input/output information signals as well as the power supply of the system. For instance, an analog chip may have to provide the power supply for an output device such as a USB device. When dealing with inductive loads, for example, it is not uncommon to require a 40–60V ESD protection structure to protect the input pads of the integrated circuit (IC) while the core of the circuit uses a 5V process. A typical inductive load situation is the pulse width modulator circuit shown in FIG. 1, which includes an inductive load 100 and a switching NMOS transistor 102. The output signal is derived by means of an operational amplifier 104 connected across a resistor 106. A free-wheeling diode 108 provides continuity for current flow when the transistor 102 switches off.
The need for a high voltage solution is becoming particularly acute in the motor vehicle industry where the number of electronic components is not only increasing but a new high voltage standard of 42V is being promoted.
Clearly high voltage technology exists to accommodate high voltage applications, however this high voltage technology is expensive and therefore not always viable, especially in low cost applications such as imaging and low cost sensors. Also, the option of using multiple chips to convert to different voltage levels is not an optimal solution.
Apart from the cost issue associated with high voltage technology, and the use of multiple chips to convert between voltage levels, an additional consideration is the ability to provide bi-directional ESD protection. One prior art bi-directional structure is the NPN triggered structure, which has a triggering and breakdown voltage of about 30V.
As pointed out above, the motor vehicle industry is moving toward a 42V standard, thus there is a need to provide a cost effective high voltage bi-directional ESD protection structure.